Method for constructing patterns in a layered manner

ABSTRACT

A process to produce models in layers is described, whereby a first material and then selectively a second material is applied in layers on a building platform and these two application stages are repeated until a desired pattern is achieved. The two materials form a solid if a suitable mixture ratio is used and the first material is a material mixture. The material mixture is at least partially prepared prior to each application stage.

CLAIM OF PRIORITY

The present application claims the benefit of the filing date ofinternation Application Serial No. PCT/DE03/01636, filed May 20, 2003(published as WO03/103932, which in turn claims the benefits of Germanapplication DE10224981.4, filed Jun. 5, 2002, the contents of which arehereby incorporated by reference in their entirety.

TECHNICAL FIELD

The invention as described here refers to a process of layer-by-layerconfiguration of patterns and/or moulds.

BACKGROUND

The application of the rapid prototyping process is well-known asstate-of-the-art technology in the production of casting moulds orcasting patterns without the use of tools.

A rapid prototyping process to configure casting patterns in particularis already familiar from DE 198 53 834 A1, for example. In this processuntreated particulate material, such as quartz sand, is applied on abuilding platform in a thin layer. Then with the aid of a sprayappliance a binding agent is sprayed onto the entire particulatematerial as thinly as possible. Subsequently a curing agent is appliedto selected areas, as a result of which designated areas of theparticulate material are solidified. After several repetitions of thisprocess an individually moulded body can be produced from the bondedparticulate material. This body is initially embedded in the surroundingunbonded particulate material and can be removed from the bed ofparticulate material after the construction process has been completed.

If, for example, in this type of rapid prototyping process a quartz sandis used as particulate material and a furan resin as binding agent, withthe aid of a sulphurous acid used as curing agent a casting mould can beproduced, which is made of the materials usually used in the mouldproduction process and therefore consists of materials known to theexpert.

With such rapid prototyping processes, as already described, first theparticulate material, then the binding agent and thereafter the curingagent must be applied. This requires a threefold application ofmaterials for every layer and is therefore very time-consuming.

There have been attempts for quite some time to eliminate at least onecoating process to shorten the production time of the pattern.

EP 0 711 213 B1, for example, describes a further rapid prototypingprocess namely selective laser sintering. In this case the particulatematerial used is croning sand, that is hot-coated foundry sand withresol or novolack resin. This means that only the particulate materialcontaining resin must be applied and the application of the bindingagent is superfluous. Traditional foundry materials can also be used inthis process and with these existing casting patterns can be producedfrom the usual materials familiar to the expert.

However, this production process also has considerable disadvantages.The resin in the sand is not completely hardened during the exposureprocess. This leads to a reduced so-called green part rigidity of theproduced moulds. The desired rigidity is reached only after removal ofthe loose sand and a subsequent oven process. Apart from the additionalprocess step in the oven there is a high risk of breakage whilede-sanding and handling the “green parts”. During the oven process anundesired distortion of the components can also occur.

Furthermore croning sands have a relatively high thermic stability,which leads to a poor de-coring capability at the relatively low castingtemperatures associated with light alloy casting.

For selective laser sintering croning sands with a higher binder contentare also necessary. The consequence of this are larger quantities of gasduring the pyrolysis of the binder while casting and therefore a higherreject risk due to blowholes in the component.

Moreover selective laser sintering in general has the disadvantage thatthe laser is a complex technique and in addition the exposure phase isalso relatively time-consuming.

Furthermore only a very limited choice of sand types and grain sizes areavailable for selective laser sintering, which means that this processis also not very flexible.

A so-called 3D printing process is familiar from the patents U.S. Pat.No. 5,204,055 and also EP 0 431 924 B1. This entails the selectiveadhesion of particulate material by the addition of binding material.This process has an advantage over selective laser sintering in that itis based on a cost-effective printing technology.

It must be said that, because of the unfavourable material properties,typical traditional foundry binders can be administered only with greattechnical effort. There is also a danger that the nozzles used tomeasure out the binding agent become clogged and unusable.

Using a drop dispenser to administer the binding agent makes the mixingof the binder in the component very poor. In order to reach comparablerigidities as with conventionally mixed sands, much higher quantities ofbinder must be added, which again leads to problems in casting becauseof the higher quantities of gas.

In PCT/DE00/03324 a further 3D printing process is revealed. This is aselective printing of particles mixed with binder with an activator, towhich a gas curing device is connected.

Again it is advantageous that here traditional foundry materials can beused.

However, gas curing is very elaborate for this process. Materials whichcreate a health hazard such as SO₂ are partly necessary, meaning that avery large amount of equipment is required and the safe operation of theapparatus becomes very costly.

As prior to the curing process not even the slightest solidification ofthe component takes place, slight displacement of the powder bed whilelayering can lead to the destruction of the entire component.

A further 3D printing process is familiar from DE 197 23 892 A1. This isa selective printing using particles coated with binder, so-calledcroning sand, with a moderating agent. This is again followed by curingwhich, according to the disclosure in this publication, takes place byway of radiation. This process also has the advantage of being able touse traditional foundry materials. However, curing of the components isvery complex in this process too, because the necessary narrow tolerancein the change of temperature requires an extensive use of equipment.

In the process published in DE 198 53 834 A1, again a 3D printingprocess, a selective printing of particles sprayed with binder withcuring agent takes place. Here too traditional foundry materials canagain be used with some degree of flexibility.

The disadvantages of this process include the complex spray applicationof the binder, inhomogeneous binder mixing and the high concentration ofbinder in the component.

Moreover, due to the formation of mist in the building chamber caused bythe spraying process, a high degree of soiling of the apparatus isconsequently caused. As a result an elaborate cleaning of the print headis necessary, as otherwise a hardening of the material on the nozzlesoccurs and causes its destruction.

Similar disadvantages are illustrated in the selective printing ofuntreated sand with binding and curing agent, as described in WO01/72502 A1.

SUMMARY OF THE INVENTION

The present invention includes a process to produce models in layers,whereby a first material and then selectively a second material isapplied in layers on a building platform and these two applicationstages are repeated until a desired pattern is achieved. The twomaterials form a solid if a suitable mixture ratio is used and the firstmaterial is a material mixture. The material mixture is at leastpartially prepared prior to each application stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows a schematic view of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Given this background situation, it is the task of the invention as setout here to make available a process whereby it is possible to carry outlayer-by-layer configuration of patterns in the most time-saving andcost-effective manner. Furthermore the process should be able to be usedindustrially based on its reliability and user-friendliness.

This task is solved by implementing a process for the configuration ofpatterns in layers, whereby using a building platform, a first materialand then selectively a second material are each applied in layers andthese two application steps are continually repeated until a desiredpattern is achieved. To this end, the two materials form a solid,provided a suitable mixture ratio is used. The first material concernedis a material mix and is at least partially prepared prior to each layerstage.

This process has proved itself to be advantageous, as it means that theprocessing times of the material mixture can be kept short and thereforethe slightly volatile ingredients in the binding agent are retained. Thepreparation can take place as required during the application process.

It would however also be possible to mix the total necessary amount ofmaterial required for the process in advance; very elaborate measureswould however have to be taken in order to prevent the vaporization ofvolatile components in the binder. Indeed, it is this highly elaborateuse of equipment that this process intends to avoid.

A further advantage of this so-called “in-process” mixing is also thegreater degree of flexibility. Firstly, only so much sand is mixed as isactually required. This means that, should the process be completedearly, there will be no unnecessary wastage. If the building process isprolonged by additional loading of components, there is no danger ofshortage of material due to the initial predetermined quantity ofmaterial. Furthermore the sand and the component materials can even bealtered during the process. Therefore, in contrast to the selectivelaser sintering process, the user does not have decide at the beginningof the process on a specific type of material for the entire buildingprocess.

A further advantage of this process in comparison with selective lasersintering is the use of cost-effective raw materials in contrast toexpensive special types of sand.

In accordance with a preferred embodiment of this invention, thematerial mixture is prepared continually. This means that the mixture isalways round about the same “age” and therefore has the same propertiesin respect of any components etc. which may have evaporated

In accordance with a further preferred embodiment of the inventionprocess, the material mixture is prepared in batches. A continual mixingprocess, as is usual in conventional moulding processes, would in factalso be possible, but because of the relatively low processing speedduring the layer configuration technically complicated.

Preferably the material mixture should consist of a particulate materialand a reactive material.

If in the invention process the second material, in accordance with afurther preferred embodiment, contains an activator, then bonding of thecomponents at room temperature by way of a chemical reaction can takeplace.

For this it would be possible for the curing of the bonded material tooccur because of a chemical reaction of the materials. Equally, curingthrough a physical reaction between the materials would also beconceivable.

Preferably with the invention process the repeated layer application andthe application of the second material takes place within the timerequired for solidification of the two materials. Thus a solidificationwithin the surface section and to the layer below and therefore a betterlayer bonding can be achieved.

Particularly good results could be achieved when, in the preparation ofthe material mixture, a residual porosity remains, as this isaccompanied by an increased gas permeability, which has a favourableeffect during casting. Moreover the second material then also reachesdeeper-lying particles, which results in better curing.

In accordance with a preferred embodiment of the invention process thesecond material is applied by means of droplet production technique.This technology has proved itself to be very accurate, reliable andsimple.

An application of the second material by using a dispensing techniquewould, however, also be conceivable.

Particularly good results could be achieved when the second material hasa carrying agent which is not involved in the curing agent reaction, aswith such a substance the wetting of the material mixture made up ofparticulate material and the first reactive components can be adjustedindependently of the chemical quantity ratios necessary for reaction.

There is still a desire to be able to process varying layer thicknesses.This also means that the addition of curing agent must be adjusted foreach layer and that preferably independently from the chosen dissolutionof the curing agent application.

For this reason a non-reacting carrying agent is mixed with the curingagent, with the help of which the required quantity ratio can beadjusted.

Preferably ethanol is used as a carrying agent. However, other alcoholscould also be used, even water is a possible alternative. Ethanol isfavourable because it is slightly volatile. If possible, however,volatilization of the entire carrying agent should have occurred priorto casting, as it can have a negative effect on the casting. During theprocess itself a large part of the ethanol already evaporates from layerto layer. The residual amount can be evaporated in a brief oven process(1 h at over 80° C.).

Ethanol also has two other positive effects. The viscosity of the agentto be dispensed is a limiting factor with the DOD (drop-on-demand)recordheads. With ethanol the viscosity of the curing agent can be reduced,which improves the functioning of the print heads.

Without a diluent a curing agent quantity to be dispensed adapted to thechemical quantity ratio would be so low that a locally severely limitedcuring process must be assumed. Moreover at this point an excessiveaddition of curing agent would take place, which would have a negativeeffect on the chemical reaction and this would in turn have negativeeffects on the rigidity of the components.

The carrying agent quantity can be calculated mathematically:

Given the desired proportional mass of the curing agent in theparticulate material is x_(h), the particulate material weight per layeris m_(s,l) then the necessary curing agent quantity m_(h) is calculatedas follows: m_(h)=m_(s,l)·x_(h)

Given additionally that the required printing resolution is r_(p) indpi, the volume of the liquid particles is v_(f,d), the area of thebuilding surface is A_(b), the density of the curing agent is ρ_(h),then the proportional volume of the carrying agent x_(t) can becalculated

$x_{t} = \frac{m_{h}}{\left( \frac{r_{p}}{0\text{,}0254} \right)^{2} \cdot v_{f,d} \cdot A_{b} \cdot \rho_{h}}$

If the particulate material used is a moulding sand such as quartz sand,silicate sand, chromite sand, zircon sand, olivine sand, chamotte sand,corundum sand or/and carbon sand, good results can be achieved with themodels. Newer materials such as synthetic sands, for example cerabeads,can demonstrate advantages in special applications and can be similarlyused. These particulate materials can be used on their own or as amixture.

Equally it would be conceivable in respect of this invention that theparticulate material is made up of a polystyrene powder, a polyamidepowder or other polymer particulate materials, or a mixture of thesepowders.

A furan resin and/or a phenol resin are particularly suitable for thefirst reactive material.

The components produced in accordance with the invention can be usedpreferably as moulds for non-ferrous castings or in the production ofinvestment patterns for non-ferrous castings.

In accordance with the invention the particulate material, preferablyquartz sand, is mixed with a small proportion of epoxy resin (binder)and in the case of furan and phenol resins with a curing agent inpredetermined quantities either in batches or mixed continually andsubsequently made into a mould. Typical mixture proportions are between0.6 and 1.8 weight-% content of resin in the quartz sand.

Conventionally the production of the mould usually takes place in amoulding machine using a pattern to achieve the moulding; production ofthe mould also partially takes place by hand. The curing, that means thebonding of the sand particles to a firm shape then takes placechemically or physically by curing the binder. The hardening process canbe supported by the use of heat.

Once the mould is finished it is then prepared for casting. Usuallyseveral mould components such as upper and lower box and cores areassembled. If required the mouldings are then sized. Then the liquidmetal in poured into the designated sprue. The high temperature of theheat melting bath leads to cracking of the resin content of the sand,especially of the outer parts nearest the heat melting bath. The gaswhich forms during this process is led away via the porosity of the sandto the exterior.

In order to avoid undesired gas cavities, the binder concentration inthe mould should be as low as possible. However, the proportion ofbinder should be adequate enough to guarantee the mechanical stabilityof the mould, even under the pressure of the metal heat melting bath.Moreover, the particles should be bonded until the metal has cooled downat least on the outer zone, and a so-called casting skin has formed.

After the metal has set, the sand should ideally trickle out of themould, if possible without the influence of any further heat applicationor mechanical aids.

The declared aim, therefore, for the production process in layers asenvisaged in the invention is to administer as little binder as possiblebut nevertheless an adequate amount in the particulate material.

Further advantageous variations of the invention as laid out here can befound in the dependent claims and the description as set out below.

To provide a more detailed explanation the invention is describedhereafter in more detail by using preferred examples of implementationand by reference to the drawing.

In the drawing the only FIGURE illustrates the pre-mix and supply of thepremixed material.

As an example, the invention process and the invention apparatus for usein a layered configuration of casting moulds made of particulatematerial, in this case foundry sand, binder and curing agent, is to bedescribed hereafter in a rapid prototyping process.

The foundry sand mixed with binder during the application stages isapplied on a platform in a thin layer (ca. 0.15-0.3 mm layer thickness).During the next stage the curing agent is selectively printed on topredetermined areas of the sand by means of a print head. This need notnecessarily be performed this way, but could also be carried out bymeans of another dosage, as for example by way of a screen printingprocess or something similar.

Wherever the curing agent penetrates into the sand, a chemical reactionis set off and the particles bond together locally, in fact only on theexact spots where curing agent was applied. No reaction takes place inthe remaining areas, Quartz sand mixed with binder therefore remainsunbonded. During the next stage the building platform is loweredcorresponding to the amount of the layer thickness and the processconsisting of application of the pre-mixed sand and pressing with curingagent on to predetermined areas begins again. This process loop isrepeated until the desired construction height has been reached and thecomponent has been completed. This now lies embedded in the uncured sandand only has to be removed from the surrounding sand.

In this example a traditional foundry resin from the family of furanresins is used as a binder. Other resins like, for example, phenolresins or also PU resins could also be used.

The particulate material combined with binder is mixed in batches duringthe building process. Here care must be taken that the batch isprocessed as quickly as possible because of the slightly volatile butreactive components in the resin. A large proportion of the resinconsists of furfuryl alcohol, which has a very high vapour pressure evenat room temperature. In order to avoid an unwanted reduction of theseconstituents in the resin, attention should be paid to the timelyprocessing of the material.

The quantity of binder can be varied and is preferably in the region of0.6-1.5 weight-% of the untreated particulate material.

The curing agent chosen is a sulphurous acid, as set out in the exampledescribed. For an optimum chemical reaction with the binder theproportion of the sulphurous acid should be in accordance with thebinder manufacturer's instructions, within a range of between 30% and 50weight % of the binder quantity. Given these binder weight proportionsthis means that ca. 0.18-0.75 weight % of the sand should be dosed.

With the invention process the percentage amounts of the curing agentpresent in the mixture have an influence which cannot be disregarded. Iftoo little curing agent is put on the sand layer previously mixed withbinder, the reaction is delayed or, if the quantity falls short of therequired minimum, the process will not begin at all.

If, by contrast, too much curing agent is introduced the component mayover-cure. In this case too the component produced rapidly loses itsrigidity.

Furthermore it is important for the curing agent to reach as many areasof contact of the particle as possible, setting off the chemicalreaction with the binder. Also a decisive factor is that the curingagent is well distributed in the particulate material. A localisedcuring agent overdosage cannot be compensated over the surface and leadsto a reduction of the rigidity.

Therefore it is important that the whole desired area is printed by thecuring agent in adequate quantities and as evenly as possible.

The quantity of curing agent must therefore be adapted to the layervolume and the binder concentration. A quantity regulation via the DODprint heads is only possible to a limited extent.

With these systems the drop size is in fact relatively rigid as it isdetermined by the configuration design of the printer. Typically thedrop diameter of the curing agent can be selected within a range of 10μm-ca. 200 μm. In our case the drops have a volume of 180 pl. Moreoverthe number of drops is determined by the desired resolution. This meansthat if the amount of curing agent added is adjusted by altering thenumber of drops, the quality of the produced components, which is to aconsiderable extent determined by the print resolution, will suffer. Ina worse-case scenario the drops must be placed so far from each otherthat the homogeneity of the curing agent added is no longer sufficientto cure the binder over the total desired surface. The rigidity of thecomponent would be considerably reduced because of this.

The problems associated with the drop size and drop quantity areincreased by the desire to be able to process varying layer thicknesses.This also means that the amount of curing agent added has to be adjustedfor each layer and this preferably independently of the chosendissolution.

For this reason a non-reactive carrying agent is mixed with the curingagent, with the aid of which the desired quantity ratios can be moreeasily adjusted. As in the example described here the non-reactivecarrying agent is ethanol.

In the example described here the quantity of carrying agent iscalculated as follows:

-   The quartz sand weight per layer is 315 g;-   The proportional mass of the binder in the quartz sand x_(b) is 1.0    weight-%;-   The proportional mass of the curing agent in the quartz sand x_(h)    is 0.5 weight-%;-   This results in a calculated curing agent quantity in the layer of    1.58 g;-   The desired print resolution r_(p) is 150 dpi,-   the drop volume v_(f,d) is 180 pl,-   the area of the building surface A_(b) is 1.125 m²,-   the density of the curing agent ρ_(h) is 1,206 kg/l,-   This results in a proportional volume of the curing agent to the    total dosage quantity of 18.5%.

It is described with reference to the FIGURE how the pre-mixing and thesupply of the pre-mixed material to the coating machine in accordancewith a preferred embodiment can take place.

As seen in the FIGURE, for this purpose a specified quantity ofuntreated particulate material 1 is taken from a so-called big bag 2 andled via a conveyor belt 3, e.g. a pneumatic conveyor, to a mixer 4. Thismixer mixes the particulate material 1 in the mixing chamber, e.g. byway of a rotating impeller as per the prescribed mix design with theresin binder and leads the batch received to a so-called receiver tank5. The receiver tank 5 is fitted with a level sensor and activates themixing process when underfilled. If in addition there is to be anelectromechanical vibrator at the receiver tank, bridging in the quartzsand, which is a frequent problem, can thus be avoided or at leastconsiderably reduced.

The particulate material pre-mixed as described, which is now slightlysticky, is conveyed via a spiral conveyer 6, depending on therequirement of the coater 7 to this piece of equipment.

This described system excels in that it is fully automated and, giventhe necessary particulate material supply, can be operated withoutinterruption in continuous operation mode.

1. A method for producing models in layers, comprising: (a) applying afirst material in a layer on a building platform wherein the firstmaterial comprises a particulate material and a first reactivecomponent; (b) selectively applying a second material to the first layervia a droplet production technique, wherein the second materialcomprises a second reactive component and a non-reacting carrying agentand calculating a quantity x_(t) of the non-reactive carrier agent to beused in the second material based upon the formula${x_{t} = \frac{m_{h}}{\left( \frac{r_{p}}{0,0254} \right)^{2} \cdot v_{f,d} \cdot A_{b} \cdot \rho_{h}}},$wherein m_(h) is quantity of the first reactive component as determinedby the product of the weight of the particulate material of the firstmaterial and a desired proportional mass of the first reactivecomponent, r_(p) is a value of resolution in dots per inch of thedroplet production technique, ν_(f,d) is a volume of liquid particles inthe second material, A_(b) is an area of the layer to recieve the secondmaterial, and ρ_(h) is the density of the second reactive component; (c)curing the first and second materials to form a solid; and (d) repeatingsteps (a) and (b) to achieve a plurality of layers to form a model,wherein the first material is a mixture of at least two components thatis at least partially prepared prior to each applying step.
 2. Themethod of claim 1, wherein the first material is prepared continually.3. The method of claim 1, wherein the first material is prepared inbatches.
 4. The method of claim 1, wherein the second reactive componentis an binder.
 5. The method of claim 1, wherein steps (a) and (b) arerepeated before the curing step is complete.
 6. The method of claim 1,wherein the first material remains porous during preparation.
 7. Themethod of claim 1, wherein the selectively applying step comprisesapplying the second material using a droplet producing technique.
 8. Themethod of claim 1, wherein the second material comprises a carryingagent.
 9. The method of claim 1, wherein the curing step comprisesforming a chemical bond between the first and second materials.
 10. Themethod of claim 1, wherein the curing step comprises forming a physicalbond between the first and second materials.
 11. The method of claim 1,wherein the particulate material comprises a moulding sand chosen fromthe group including quartz sands, zircon sands, olivine sands, chamottesands, and combinations thereof.
 12. The method of claim 1, wherein theparticulate material comprises a polystyrene powder, a polyamide powder,and combinations thereof.
 13. The method of claim 1, wherein the firstreactive component comprises furan resin, polyurethane resin, andcombinations thereof.
 14. The method of claim 1, wherein the modelcomprises a mould for a non-ferrous casting.
 15. The method of claim 1,wherein the model comprises an investment pattern for a non-ferrouscasting.
 16. A method for producing models in layers, comprising: (a)applying a first material mixture in a layer on a building platformwherein the first material mixture comprises a quartz sand and a epoxyresin, wherein the epoxy resin is present between about 0.6 and 1.8% ofthe total weight of in the first material mixture; (b) selectivelyapplying a second material mixture to the first layer via a dropletproduction technique, wherein the second material comprises a secondreactive component and a non-reacting carrying agent, wherein a quantityx_(t) of non-reactive carrier agent to be used in the second material iscalculating based upon the formula${x_{t} = \frac{m_{h}}{\left( \frac{r_{p}}{0,0254} \right)^{2} \cdot v_{f,d} \cdot A_{b} \cdot \rho_{h}}},$wherein m_(h) is quantity of the epoxy resin as determined by theproduct of the weight of the quartz sand of the first material and adesired proportional mass of the epoxy resin, r_(p) is a value ofresolution in dots per inch of the droplet production technique, ν_(f,d)is a volume of liquid particles in the second material, A_(b) is an areaof the layer to recieve the second material, and ρ_(h) is the density ofthe second reactive component; (d) curing the first and second materialsto form a solid; and (e) repeating steps (a) through (c) to achieve aplurality of layers to form a model, wherein the first material mixtureis at least partially prepared prior to each applying step.